The present invention relates to a wireless communication method, and particularly to a wireless communication method in which in a wireless system to perform cellular communication, an array antenna type wireless apparatus including a plurality of antennas is used, beams are formed in time division and packet transmission is performed.
Antennas used in cellular system base stations are directional antennas to form sectors, and in some antennas, an array antenna to further subdivide the sector is not added. The respective base stations use the same frequency channel and the mutual communication causes interference. The base stations transmit pilot signals in, for example, a nondirectional pattern, and a wireless terminal receives these signals and measures the respective signal levels. In the wireless terminal, a C/I (ratio of carrier wave power to interference wave power) can be calculated from the measured signal levels. The wireless terminal calculates a transmission data rate of the forward-link from the calculated C/I. The calculated transmission data rate is transmitted to the nearest base station via wireless. Based on this information, the base station specifies the data rate selected by the wireless terminal among a plurality of prepared modulators, and modulates user information sent from a network. The modulated signal is transmitted as a wireless signal from the antenna of the base station using the same radiation pattern (for example, nondirectional pattern) as the pilot signal.
As an example of the cellular system, a system of cdma2000 1x evolution-data only (1xEV-DO) will be considered. The detailed specifications for this system are obtained in, for example, non-patent document 1. In this system, a pilot channel and a data channel transmitted from a base station are multiplexed in time-division. A wireless terminal sequentially calculates a transmission data rate of a forward-link from the C/I of the time-division multiplexed pilot signal, and sequentially requests the data rate value from the base station. On the other hand, radiation pattern formation by an array antenna, and a forward-link data rate determination method in the system using that are not described in the form of specifications in the non-patent document 1 and other documents as well.
In the cdma2000 1xEV-DO system, there is disclosed a system of determining a forward-link data rate in a base station system using an array antenna (see, for example, patent document 1). In the disclosed system, for example, a sector is divided in narrow beam patterns using the array antenna, an interference power is measured from pilot signals transmitted in the plurality of beam patterns to estimate a C/I, and the forward-link data rate is obtained. Incidentally, the application of patent document 2 is a divisional application of the patent document 1 and discloses the same technique.
Besides, there is disclosed a system in which in fixed wireless, Signal Interference Ratio (SIR) information is shared between base stations to avoid interference (see, for example, patent document 3). In the disclosed method, for example, at the time of start of communication, or by a periodic operation, the interference is estimated at the reception side of each communication machine (base station and terminal). Alternatively, the signal ratio of signal to total interference signal is estimated. A database including data indicating the mutual interferences is installed, and each communication machine stores the interference information into the database. The interference in the relevant time slot is determined by accessing the database from the base station, a slot with a sufficiently low interference is selected and communication is performed. In the case where an intense interference is estimated in all slots, an assignment is not made to a slot.
Besides, another method is disclosed in patent document 4. In the disclosed system, a plurality of base stations uses array antennas, and transmits narrow beam pattern pilot signals. A wireless terminal receives the pilot signals transmitted from the respective base stations, estimates transmission paths of those, and transmits them as transmission path information to the base stations. The base station uses the transmission path information received from the wireless terminal and scheduling information shared with peripheral base stations to estimate forward-link signal quality, and determines a forward-link transmission rate to the wireless terminal. The forward-link transmission rate is used, and a data packet is transmitted in the narrow beam pattern directed to the wireless terminal.
[Patent document 1] JP-A-2003-338803
[Patent document 2] JP-A-2005-143148
[Patent document 3] JP-A-10-155181
[Patent document 4] JP-A-2003-304577
[Non-patent document 1] The Third Generation Partnership Project 2 (3GPP2) Specifications, [online], C. 50024-v4.0 cdma2000 High Rate Packet Data Air Interface Specification, [retrieval on Feb. 13, 2006],